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Healthcare data shows that about one in three people suffers some type of chronic illness. The word "chronic" has a specific medical meaning and refers to any condition that lasts for more than a year, requires routine checkups and limits physical activity in some way. So, what's the problem? Sadly, most sufferers of these invisible conditions, whether illnesses like rheumatoid arthritis or epilepsy, are often treated with indifference or outright accused of faking.

What are the biggest issues for sufferers, what can others do to help and what are some of the most common invisible conditions? Here's a quick rundown you can refer to as a way to educate yourself about invisible illnesses and disabilities:

What's the Problem?

People who suffer from non-apparent conditions like heart disease, various kinds of cancer and cystic fibrosis, for example, are often thought to be perfectly healthy. As a result, if they become ill and require emergency help, people are often slow to respond. "She looks so healthy. Are you sure we really need to call an ambulance?" is a common type of response when someone with an invisible illness loses consciousness in public.

Another issue has to do with public scorn or dismissal. When people with invisible conditions miss work or need to sleep longer than normal, they often come in for derision and hurtful rumors from co-workers and acquaintances. "Why were you off work for so long? You look great?", "You have what? I never knew that was something people went to the hospital for," and "Maybe if you just eat right and get more exercise, you'll feel better," are a few of the most common phrases heard by sufferers. This kind of social stigma or downright ignoring the condition can take an emotional toll on people who are already quite ill.

What to Say and What Not to Say

Some of the least helpful things to say to a person you think might have an invisible condition include comments like, "Hey, you look great. Why can't you join us for lunch?", "It's all just in your head," "You know what? You're probably taking too much medication," "You don't look very sick to me," "You really should try to have a more positive attitude," and "Hey, are you still feeling bad?"

Fortunately, you can avoid saying things like that and instead offer help by saying things like, "How are you feeling?", "Is there anything I can do to help?", "I'm sorry you're having to endure this," and "Let me know if I can ever be of help in any way."

The Most Common Invisible Conditions

There are hundreds, perhaps thousands, of invisible conditions, as posted by this residential elevators company. Some are deadly diseases that have few outward physical manifestations. Others, while not potentially fatal, are still among the most serious medical maladies a person can endure. Invisible illnesses include conditions like cancer, arthritis, allergies, digestive problems, mental illness, diabetes, lupus, neurological conditions, Lyme disease, Alzheimer's disease, heart conditions, multiple sclerosis and more.

Some doctors distinguish between invisible illnesses and invisible disabilities, but there is a lot of overlap between the two sets of conditions. You'll often hear about "non-apparent" disabilities like AIDS, traumatic brain injury, ADD and pretty much any chronically disabling condition that is not obvious to an ordinary observer.